Vertical Cavity Surface Emitting Laser (VCSEL) diodes are typically utilized to transmit digital data over optical fibers. Driver circuits are utilized to deliver current through the VCSEL diode. When 0s are being transmitted, a threshold current is driven through the VCSEL diode. This threshold current is referred to by those skilled in the art as the bias current. When 1s are transmitted, a larger current than the bias current is passed through the VCSEL diode, the additional amount of current being referred to as modulation current.
A problem with driver circuits known to the art is the lack of headroom voltage for a current source. Typical VCSEL diodes are modeled as forward voltage of around 1.6 volts with a series resistance of approximately 20 Ohms. With a maximum amount of current allowed to flow through the diode at 20 milliAmperes, the voltage drop across the diode is approximately 2 Volts. A one-volt voltage drop occurs at a transistor when in the forward active region. In driver circuits with a power supply of about three volts known to the art, a two-volt voltage drop across the diode and a one-volt voltage drop across the transistor does not permit any headroom voltage for a current source placed in series with the diode and transistor. A method to ameliorate the lack of headroom voltage in three-volt driver circuits known to the art is to provide alternating current (AC) coupling. AC coupling requires external components which may not be suitable for placement upon a chip, such as inductors and capacitors.
Consequently, it would be advantageous if a system and method of driving a semiconductor diode existed which could provide additional headroom voltage for low voltage supplies. It would also be advantageous if a system and method existed which could drive a semiconductor diode with a low voltage supply without AC coupling.